Fix:remove a img close which can not operate (#9267)

### What problem does this PR solve?


https://github.com/infiniflow/ragflow/issues/9149#issuecomment-3157129587

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/nlp/__init__.py

         pixel_data1 = img1.tobytes()
         pixel_data2 = img2.tobytes()
         if pixel_data1 == pixel_data2:
-            img2.close()
             return img1
 
     width1, height1 = img1.size
 
     new_image.paste(img1, (0, 0))
     new_image.paste(img2, (0, height1))
-    img1.close()
-    img2.close()
     return new_image